Automotive safety critical applications, such as emergency braking to prevent a crash, require that the CMOS imager runs self tests at regular intervals during operation to detect any failure mechanism that may occur due to reliability issues. As other examples, imagers are used as sensors in lane tracking systems, edge-of-road detection systems, near-obstacle-avoidance detection systems, crash alert systems and passenger position sensing systems. Such safety critical applications require imagers with high reliability.
Conventional imagers are tested at the manufacturer using precision equipment to inject a known quantity of light and compare the imager output to the expected output resulting from the known quantity of light. Such test equipment is generally sophisticated and correspondingly expensive. Additionally, operational testing is performed at the manufacturer using precision equipment to inject a known quantity of light and compare the imager output to the expected output resulting from the known quantity of light. Such test equipment is generally sophisticated and correspondingly expensive.
Camera systems for use in safety critical applications can be designed based on a “majority vote” among three cameras viewing approximately the same scene. As an example, one of three cameras detects an object interpreted by its image processing software as the headlights of an approaching car on a collision path. Since no such object is detected in either of the two other cameras, no further action is required and the images for the other two cameras are used for image processing. The failing camera unit results in an error message “camera service required immediately”. Such a “majority vote” camera system has a relative large latency, before a pixel error or a pixel cluster error is detected by the software in the camera system. This is so, because pixel cluster defect errors occurring over the product's lifetime must interfere with the detection of relevant objects in the image to trigger an alarm.
In contrast, a less expensive two-camera system for use in safety critical application may be designed based on active real-time validation of pixel output circuits. As will be described, the present invention uses a column-parallel analog-to-digital-converter (ADC) architecture of an image sensor. The present invention provides complete testing of the signal path from each pixel in a row and throughout the column-parallel ADC architecture. In addition, the test is run during real time image processing, without losing real data or frame rate.